1. Field of the Invention
The present invention relates to a switching power supply and a semiconductor device used therefor. More particularly, the present invention relates to a novel control system that can reduce the size as well as noise of a switching power supply.
2. Description of Related Art
In general, a switching power supply is provided with a switching transformer, a switching element connected in series with a primary winding of the switching transformer; a control circuit that controls ON/OFF of the switching element; a rectifier/smoothing circuit connected with a secondary winding of the switching transformer; and an output voltage detection circuit that feeds a detection voltage, changing in accordance with an output voltage of the rectifier/smoothing circuit, back to the control circuit (see JP 3480462B, for example).
FIG. 1 is a circuit diagram showing an exemplary configuration of a conventional switching power supply. This switching power supply is provided with a switching transformer 101 having a primary winding 101a, a secondary winding 101b and an auxiliary winding 101c. A switching element 102 is connected in series with the primary winding 101a, and a direct current (DC) voltage Vin is applied to the primary winding 101a and the switching element 102. The switching element 102 is subjected to the ON/OFF control (PWM control) by a control circuit 103, thus conveying electric power from the primary winding 101a to the secondary winding 101b of the switching transformer 101.
A voltage generated at the secondary winding 101b of the switching transformer 101 is rectified and smoothed by a rectifier/smoothing circuit 104 composed of a diode 104a and a capacitor 104b into an output DC voltage, which is fed to a load 105. This output DC voltage is detected by an output DC voltage detection circuit 106, and the detected voltage is fed back to the control circuit 103. The thus detected and fed-back voltage is then given to an ON duration control circuit 103a of the control circuit 103, and an output thereof is given to a pulse control circuit 103b. An output of the pulse control circuit 103b is given to a drive circuit 103c, and an output of the drive circuit 103c serves as a control signal (gate signal) of the switching element 102.
A voltage generated at the auxiliary winding 101c of the switching transformer 101 is rectified and smoothed by an output smoothing circuit 107 composed of a diode 107a and a capacitor 107b, which then is fed to the control circuit 103 as an operating voltage Vcc. Incidentally, at the time of starting prior to the ON/OFF control by the switching transformer 101, the operating voltage Vcc is fed from the DC voltage Vin through a resistor 108.
The voltage generated at the auxiliary winding 101c further is input to a ringing generation circuit 109 composed of a diode 109a, a resistor 109b, a capacitor 109c and a resistor 109d, and an output of the ringing generation circuit 109 is input to a comparator circuit 103d of the control circuit 103. A comparator CMP of the comparator circuit 103d makes a comparison of the input voltage with a reference voltage Vref, and an output of the comparator is given to the drive circuit 103c by way of a bottom counter circuit 103e and a delay circuit 103f. The ringing generation circuit 109, the comparator 103d, the bottom counter 103e and the delay circuit 103f are configured to generate a delay signal based on the number of the ringing occurring at the transformer during an OFF period of the switching element and makes the drive circuit 103c to delay an ON timing of the switching element in accordance with this delay signal. As the load changes from a heavy one to a light one, the number of the ringing increases. Thus, in accordance with the thus increasing number of ringing, the timing to turn ON the switching element can be delayed so as to increase an oscillation period of the switching element. In this way, as the load becomes lighter, the switching frequency can be made lower, thus obtaining the effect of decreasing a switching loss.
In the case where the primary side and the secondary side of the switching transformer have to be separated electrically, a photocoupler may be used in the output DC voltage detection circuit 106 to convey a fed-back signal to the control circuit 103.
The above-stated conventional switching power supply requires an output voltage detection circuit that is connected with the secondary side in order to detect an output voltage and feed back the same to a control circuit, which makes it difficult to downsize a switching power supply. Especially when the primary side and the secondary side are to be separated electrically using a photocoupler, it is difficult to downsize the switching power supply.
Furthermore, by virtue of the functions of the ringing generation circuit 109, the comparator circuit 103d, the bottom counter circuit 103e and the delay circuit 103f, the above-stated effect of suppressing a switching loss for a light load can be obtained. However, no countermeasures have been taken for reducing a switching noise.
Moreover, according to the conventional switching power supply, as a state of the load on the output side changes, the operating frequency of the switching element will change significantly. To cope with this, the size of the switching transformer 101 will increase, which makes it difficult to downsize a switching power supply.